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Pressure

This information applies to pressure measurement in fluids, such as air, other gasses, and water, as required in a variety of buildings research applications.

Pressure measurements fall into three categories, depending on what the measured pressure is compared to:

  • Absolute pressure—the pressure of the fluid compared to a vacuum with no pressure
  • Gauge pressure—the pressure of a fluid compared to the air pressure in the immediate surrounding environment
  • Differential pressure—the difference between fluid pressures at two locations as determined by particular measurement needs. Differential pressure measurements always take the form of P differential = P (location a) - P (location b). When measuring differential pressures, it is critical to be clear which measurement location is a and which is b. This is especially important when the higher pressure can occur at either a or b (e.g., when measuring pressures across a building enclosure). In such cases, the sign of the result is critical information and cannot be assumed. We recommend using statements like "Pressure a wrt (with respect to) Pressure b" or "Pressure a - Pressure b".

The pressure of a fluid independent of any flow is the "static pressure". A sensor placed so that the moving fluid flow impinges on it will sense an additional pressure ("dynamic pressure") related to the momentum of the flow. See the application notes for guidelines on measuring static and dynamic pressure.

Common units of pressure include Pascals (Pa) for air pressure, inches of water (in H2O) for gas pressure and air pressure, and pounds per square inch (psi) for refrigerants, water, and atmospheric pressures.

Table—Common building research pressure measurement categories and applications
CategoryWhy measured?Differential, gauge, or absoluteTypical rangeNotes
Building internal operating pressure (Poutdoor – Pindoor)For investigating the pressurization effects of ventilation or HVAC fans, or for characterizing likelihood of depressurization that may cause combustion products spillage. Possibly, for quantifying infiltration driving forces.Diff-10 to +10 PaDirect exposure to wind may yield much higher values
Atmospheric combustion appliance draft pressure (Pvent-Proom)As an indicator of vent spillage (“backdrafting”) and (though not absolute) of direction of vent flowDiff-50 to +20 PaUsually measured as vent wrt room
HVAC system pressures (Pduct – Proom, or Pduct1-Pduct2)For estimation of blower flow rates, characterizing system pressure drops, or estimating leakage ratesDiff-300 to +300 PaNote distinction between static and dynamic pressure
Refrigerant line pressureTo establish operating conditions of compression cycle refrigeration systemGauge0 to 500 psi (or higher)Requires knowledge of compression refrigeration systems
Water line pressureFor estimation of flow rate through a valve or system of known characteristics, or to screen for adequate line pressure in a systemGauge15 to 150 psiRequires “wet” design (internals exposed to water)
Gas line pressureTo check proper adjustment and operation, or for estimation of flow rate through a device of known characteristics (e.g., a pilot light orifice)GaugeUp to 15 inches of waterWarning: hazard of fire and explosion
Barometric pressureTo calculate exact density of air, sometimes needed for precise calculations of fan flow rate, energy transfer, and relative humidityAbsolute14 to 15 psi 

Note: always select sensors that can withstand the maximum pressure that can be expected in your application.

Sensor and transducer types

Pressure transducers are widely available from many manufacturers, with linear (analog) voltage output or 4 to 20 mA (analog) current output. Some come equipped with visual displays (most often digital LCD displays).

Pressure transducers in general may be sensitive to temperature changes. Consult manufacturer's specifications for model-specific information.

Low-Level Differential Pressure

We use the term "low-level" for pressure transducers with full-scale ranges of about 25 pascal (Pa) to 500 Pa. Differential pressure transducers are either uni-directional or bi-directional. Uni-directional models provide an output only when the “high” pressure input is at a higher pressure than the “low” pressure input. Bi-directional models can provide an output with either input at a higher pressure. The fittings on low-level pressure transducers are most often barbed fittings that are compatible with flexible tubing (or "hose") of the correct size. PVC and urethane are common tubing materials used in these applications.

Refrigerant Line Pressure

Refrigerant line pressure measurement can be used in establishing the operating conditions of a compression-cycle heat pump or air conditioner, and in determining whether the system may have a refrigerant over-charge or under-charge. Refrigerant pressures may range up to 500 psi and even higher for some refrigerants (e.g., up to 800 psi for R-410a).

Water Line Pressure

Water line pressure measurement may be useful in estimating flow rates or checking the pressure drop in domestic water systems. Typical water pressure in buildings on a municipal system is about 50 to 110 psi. Extremes may be lower and higher. Typical water pressure in buildings on private wells is about 30 to 50 psi.

Gas Line Pressure

Pressure measurement in fuel gas systems may be of value in estimating flow rates through pilot light orifices or other components, or in validating adequate pressure during operation.

Barometric Pressure

Barometric Pressure is used in detailed psychrometric calculations, and in calculation of the density of air, which can be important in accurate determination of heat transport via airflow. Barometric pressure is by definition an absolute pressure measurement.